A primary function of a wireless receiver is to down-convert the received wanted radio frequency signal to a baseband and/or digital form in order to process the wanted signal. In order to selectively extract the wanted signal from noise or other undesired signals, filters are used. Filters are employed at various stages of a receiver's architecture, from analog radio frequency (RF) filters through to digital filters. A digital filter operates on a discrete time sample set, where the value of the samples has been rounded to the nearest value from a finite set of possible values, typically represented as a binary number.
An analog discrete time filter (DTF) operates on a discrete time sample set, where the value of the samples is continuous (typically a real number), and where digitization occurs in an analog-to-digital converter (ADC) located after this filter. DTFs have two forms, i.e. Finite Impulse Response (FIR) and Infinite Impulse Response (IIR). In receivers having FIR DTFs, it is known that sampling capacitances need to be accurately matched.
Referring now to FIG. 1, a known example of a decimate-by-2 FIR discrete time filter 100 is illustrated. The decimate-by-2 FIR discrete time filter 100 includes a source voltage 110 coupled via a source resistance 112 to an input port 114 receiving an input voltage.
A full filter is made of N-branches 116, but each branch does not form a full filter by itself, just a sampling capacitor Cs or, in general, a set of sampling capacitors Cs. Control of the N branches of FIR DTFs is typically implemented by means of transistor switches with small conducting resistances RON. In this manner, sampling capacitors Cs 126 in each branch are selectively coupled to the input by a set of respective switches (ϕ0, ϕ1, ϕ2) 122. The sampling capacitors Cs 126 in each branch are selectively coupled to the output by a further set of respective switches (α0, α1, α2) 128. The sampling capacitors Cs 126 in each branch are selectively reset by a further set of respective switches (θ0, θ1, θ2) 124. The outputs from the N-branches 116 are then combined (summed) and coupled to an output capacitor (Cout) 120 to provide an output voltage Vout from the decimate-by-2 FIR discrete time filter 100. The charge of the output capacitor (Cout) 120 is selectively maintained or discharged by means of an output capacitor reset switch 118 (β0).
A simplified representation of the output side of the decimate-by-2 FIR discrete time filter 100 during the time interval during which two of the three output switches are closed is illustrated at 150, with a sampling capacitor Cs 126 in each of two out of three branches being coupled to the output capacitor (Cout) 120 by respective small conducting resistances RON 152 that represent closed transistor switches. A full representation of the filter during the time interval in question would include the third capacitor being connected to the input.
However, DTFs are known to suffer from a number of problems. For example, in the process of constructing the output signal sample through charge sharing between sampling capacitors Cs 126 and output capacitor (Cout) 120, these small RONs 152, together with the sampling and output capacitors, form a circuit with a wide noise bandwidth, substantially larger than the output sampling frequency. This results in substantial noise aliasing.
The time period described above and represented in 150 of FIG. 1 corresponds, to a timing interval [0, ½*1/fs], with all other time intervals during which any two from amongst the switches 128 are open. The switches are controlled by signals α0, α1, α2. During these time periods, only the output switches' noise is added to the signal. The full operation of the filter is, however, composed of additional phases. During these additional phases, additional noise sources add noise on top of the signal, e.g. noise generated by the source resistor Rs 112, the input switches RON φ122 and the reset switches RON θ124 is also added to the signal. In decimating DTF the noise added by the output switches is often the dominant noise contribution because of the lower output sampling frequency.
Furthermore, in transferring the charge from multiple sampling capacitances 126 to an output capacitance, only part of the total charge ‘Q’ stored on the sampling capacitors 126 is transferred to the output capacitor Cout 120 in passive DTFs of the type illustrated in FIG. 1, sometimes referred to as ‘charge redistribution’. This results in attenuation of the output voltage. For example, in a decimation-by-2 FIR filter, this occurs by averaging, say, 2 successive samples at a time. The maximum magnitude of the voltage transfer function (TF) is, thus:Max(|TF(f)|)=2Cs/(2Cs+Cout).  [1]
As a consequence of the significant signal attenuation, and increased noise levels, DTFs are currently only used in baseband processing, i.e. once signal amplification/noise reduction has been performed, so that this inherent signal attenuation and reduction in noise can be tolerated.
Thus, a need exists for an improved DTF design that may be more tolerant of, or address, noise levels and/or signal attenuation.